How injecting CO2 enhances marine natural gas hydrate exploitation: Review and prospect

Natural gas hydrate (NGH) is a clean energy source with huge resource volumes. How to exploit NGH efficiently, economically, and safely has recently generated interest. CO2 hydrates can form under milder conditions than NGH, so injecting CO2 to assist NGH exploitation has been extensively researched. Based on this, this review discussed how injecting CO2 enhances marine NGH exploitation and presented a win-win strategy for clean energy access and carbon storage. We first demonstrated that CO2 hydrates can form in marine NGH depositional environments, and then summarized three modes of CO2 injection, and innovatively studied the applicability of NGH stimulation principles to hydrate-based CO2 sequestration. The review mainly revealed that (i) the phase transformation of CO2 in NGH reservoirs is affected by temperature, pressure, minerals, and salinity, and liquid CO2 can form mixed hydrates with residual CH4 to achieve long-term stable storage. (ii) CO2 replacement, CO2 hydrate cap, and carbon sequestration in depleted reservoirs are advanced coupling modes. Considering the accumulation characteristics of most marine reservoirs, it may be more reliable to choose injecting CO2 or CO2/N2 after exploitation. (iii) The principle of NGH enhancement measures is beneficial to CO2 hydrate formation: Low temperature caused by depressurization can increase the driving force of CO2 hydrate formation; Expanding the contact area between wellbores and reservoirs allows more CO2 injecting; Improving the reservoir permeability is conducive to heat and mass transfer. Multi-gas source extraction can enrich the storage space of CO2. These key findings can help us understand these coupling modes more deeply.